The present invention relates to a method for preparation of olefin oligomers, and particularly relates to a method for preparation of olefin oligomers which can perfectly prevent halogen from being present in the polymerization product and provide olefin oligomers at a high yield. More particularly, the present invention is concerned with a method for preparation of olefin oligomers which is capable of optionally regulating the viscosity of the oligomers without any bad influence on the viscosity index thereof.
As the well-known methods for polymerization of olefins, there are a method employing Lewis acid catalyst such as aluminum chloride, a method employing a mixture of aluminum chloride and metallic aluminum powder or metallic zinc powder as the catalyst etc. There is also known a method of effecting polymerization by employing a liquid catalyst prepared by dissolving excess aluminum halide in a complex consisting of aluminum halide and ethyl acetate (molar ratio = 1:1) (cf. Japanese Patent Publication No. 3804/1969). However, all of these methods are defective in that they can produce no more than a polyolefin oil having a relatively high viscosity under the ordinary reaction conditions, and in the case where the reaction temperature is elevated with a view to preparing a low-viscosity polyolefin oil, not only the viscosity index of the resulting polyolefin oil will be reduced but also the yield thereof will decrease.
Another trouble with the conventional methods for polymerization of olefin, is that the halogen component of the catalyst is present in the product polyolefin oils. The presence of halogen in the product has a bad influence of grave importance on the process of after-treatment of the polymerization product. For instance, at the time of distilling the monomer and/or dimer from the polymerization product, there takes place the thermal cracking of a part of the halogen in the polyolefin oil thereby to generate hydrogen chloride, so that the distillation apparatus is corroded thereby. Furthermore, at the time of the hydrogenation of the double bonds remaining in the polymerization product carried out for the purpose of improving the heat stability as well as the oxidation stability of said product, there occurs the trouble that the hydrogenation catalyst is rendered inactive by the halogen.